In a large heavy saucepan, stir the sugar and cornstarch to blend. Whisk in the cream and milk and cook over medium heat, whisking constantly, about 10 minutes, or until the mixture thickens and begins to bubble.
Transfer the cream mixture to a heatproof bowl and add the mint. Set the bowl over a bowl of ice water to cool the mixture down quickly and stir occasionally until cool; discard the mint after 5 minutes. (Don't steep the mint for too long or it will impart a grassy flavor; a quick steep while the cream cools will do.) Cover the mint cream and refrigerate about 4 hours, or until completely cold.
Freeze the mint cream in an ice cream maker according to the manufacturer's instructions until frozen.
Meanwhile, place the chocolate in a heatproof bowl, set it over a saucepan of simmering water and stir the chocolate until it has just melted. Remove from the heat and cool until the chocolate is barely warm and still fluid.
Transfer the gelato to a large bowl set over a bowl of ice. Working quickly, drizzle the melted chocolate into the gelato while whisking or stirring vigorously. The chocolate will flake apart as it hits the cold gelato, forming bits of chocolate throughout. If you prefer larger pieces of chocolate throughout the gelato, use a wooden spoon to slowly stir in the melted chocolate. Serve immediately, or transfer to a container and freeze until firm, 3 to 4 hours.
Curtis serves his homemade gelato between waffle cookies.
Photograph by Ray Kachatorian
Recipe from Good Food, Good Life: 130 Simple Recipe You'll Love to Make and Eat. Copyright (c) 2015 by Curtis Stone. By arrangement with Ballantine Books.